High temperature thermistor composition

ABSTRACT

A high temperature thermistor composition comprising a spinel-type Mg Al 2  O 4  and perovskite-type La CrO 3  or (La, Sr) CrO 3  is usable for the temperature range from 400° to 600° C, and a high temperature thermistor composition comprising spinel-type solid solution Mg(Al, Cr) 2  O 4  and perovskite-type solid solution (La, Sr) CrO 3  or LaCrO 3  is usable for a higher temperature range up to 1000° C.

BACKGROUND OF THE INVENTION

This invention relates to compositions for thermistor, and moreparticularly to compositions for high temperature thermistors.

The conventional thermistors comprise, for example, cobalt oxide,manganese oxide, nickel oxide and/or iron oxide, and they have beenusually employed at a temperature range from room temperature to about300° C. Recently, a high temperature thermistor has been developed whichis available for employment in a higher temperature range i.e. up toabout 1000° C. For example, Japanese patent publication No.41-15855/1966 discloses a high temperature thermistor consisting of SnO₂--TiO₂ solid solution as a principal component and as additives, atleast one oxide selected from the group consisting of Sb₂ O₃ and Ta₂ O₅in an amount of less than 5 % by mole, which can be used at a hightemperature up to 1000° C. It is also disclosed in Rev. of ScientificInst., 40, pp544-549(1969) that a thermistor consisting of ZrO₂ as aprincipal component and an additive of Y₂ O₃ is available for employmentat a high temperature i.e. up to about 1000° C. Furthermore, U.S. Pat.No. 3,598,764 discloses a thermistor consisting of CeO₂ as a principalcomponent and an additive of ZrO₂ in an amount of 2 to 15 % by weight,which is available for employment at a high temperature i.e. up to 1000°C. However, these conventional high temperature thermistor compositionshave some disadvantages such at time-dependent characteristics.Particularly, they show unstability when they are used at a hightemperature under applied d.c. voltage.

Recently, a high temperature thermistor which can be used in a workingtemperature above 300° C has been intensively required. For example, athermistor is desired which can be used as a sensor of a temperaturecontroller in a thermal reactor where exhaust gas from a motor carengine is reburnt. In this case, this thermistor must operate steadilyat a temperature range from 600° C to 1000° C, and in addition the hightemperature thermistor is used under an application of d.c. voltagebecause a d.c. electrical battery used as an electric power source in amotor car.

SUMMARY OF THE INVENTION

Therefore, an object of the present invention is to provide a novel andimproved high temperature thermistor composition.

Another object of the invention is to provide an improved hightemperature thermistor having superior characteristics such as goodreproducibility and small resistance deviation at a given hightemperature under load aging with applied d.c. voltage, with variousvalues of resistivities and B-values (defined hereinafter) at workingtemperature.

A further object of the invention is to provide two kinds of hightemperature thermistor compositions which are suitable for use at atemperature range from 400° to 600° C and 600° to 1000° C, respectively.

These objects of the invention are achieved by providing a hightemperature thermistor composition according to the present invention,which comprises 10 to 90 mole % of spinel-type MgAl₂ O₄ and 10 to 90mole % of perovskite-type LaCrO₃.

Especially, a thermistor suitable for use at a high temperature from400° to 600° C is provided by a composition of spinel-type MgAl₂ O₄ andperovskite-type LaCrO₃ or (La, Sr) CrO₃ solid solution, and a thermistorsuitable for use at a higher temperature from 600° to 1000° C isprovided by a composition of spinel-type solid solution Mg(Al, Cr)₂ O₄and perovskite-type LaCrO₃ or (La, Sr)CrO₃ solid solution.

These thermistors show good reproducibility, and small resistivitydeviation even after use for a long time at working temperature under anapplication of d.c. voltage. The resistance and B-value at the workingtemperature of these thermistors can be changed widely by changing amolar ratio of spinel type and perovskite-type phases. For thethermistor for use at the 600° to 1000° C temperature range, thesevalues can be changed further by changing the Al/Cr atomic ratio inspinel-type Mg(Al, Cr)₂ O₄.

DETAILED DESCRIPTION OF THE INVENTION

A thermistor produced according to the invention is described withreference to FIG. 1.

In FIG. 1, reference numeral 1001 designates a rod-type sintered body ofa high temperature thermistor having a composition according to theinvention. A pair of electrodes 1002 made of platinum is set, beforesintering, into the pressed body by a pressing process.

It has been discovered according to the present invention that by usinga sintered body comprising a spinel-type Mg Al₂ O₄ and perovskite-typeLaCrO₄ or (La, Sr) CrO₄ solid solutions, there is provided a hightemperature thermistor available for use at a temperature range from400° to 600° C with a high stability of electrical characteristicsthereof in load aging under an application of a d.c. voltage. Thepreferred composition of this thermistor is formed from a secondarysystem of 10 to 90 mole percent of MgAl₂ O₄ and 10 to 90 mole percent ofLaCrO₃. In LaCrO₃, La ion can be substituted partially by Sr ion.

A spinel-type MgAl₂ O₄ has a high resistivity, and perovskite-typeLaCrO₃ has a low resistivity. Further, the latter has a smalltemperature coefficient of resistivity. However, it is found that themixed ceramic body of these two components provides a high temperaturethermistor with moderate resistivity and B-value at working temperatureand furthermore with superior stability of electrical characteristicsthereof in load aging under an application of d.c. voltage. Moreover,such a mixed ceramic thermistor of this secondary system has anadvantage that the resistance and B-value can be desirably chosen for aworking temperature by changing just a ratio of each component withoutchanging the composition.

The high temperature thermistors according to the invention are preparedas follows. Starting ingredients of MgO, Al₂ O₃, La₂ O₃, Cr₂ O₃ andSrCO₃ in appropriate proportion are intimately mixed in a ball mill withagate balls under alcohol. After mixing, the mixture is dried and thenprefired at a temperature of 1100° C to 1400° C for 1 to 5 hours. Theprefired mixture is then subjected to wet grinding in the ball mill for10 to 24 hours, and dried. The pulverized prefired powder is then mixedwith pure water by 6% in weight and granulated. After that, the powderis pressed into a rod form of 2mm in diameter and 4 mm in length withtwo platinum lead wires of 0.3 mm in diameter inserted into the rod-typepowder compact as shown in FIG. 1. The platinum electrodes 1002 are setinto the powder compact 2 mm in depth by a pressing process. Finally,the pressed rods with Pt lead wires are sintered at a temperature of1500° to 1700° C in air for 1 to 5 hours.

For the obtained rod-type thermistors of the invention, electricalresistances were measured in a temperature range from 20° to 1000° C,and B-values were calculated from a following equation: ##EQU1## whereR₁ and R₂ are electrical resistances at temperatures of T₁ ° K and T₂ °K, respectively, and B is a constant, that is so-called B-value of athermistor. Furthermore, aging tests of electrical resistance werecarried out as it is one of important factors for a practical use.Resistance deviation in an aging test is defined by a term of (R_(t)-R_(O) /R_(O)) × 100 (%), where R_(O) is an initial resistance in anaging test and Rt is a resistance after a lapse of time t, and theelectrical resistance was measured at 500° C under a continuousapplication of d.c. voltage of 6V.

Table 1 shows the various compositions of the high temperaturethermistor which is available for use at 400° to 600° C temperaturerange and the results of their measured electrical characterics and theload aging test of 1000 hours under applied d.c. voltage of 6V. Forsamples No. 1 to No. 13, these results are shown for various molarratios of spinel-type MgAl₂ O₄ and perovskite-type LaCrO₃. As understoodfrom Table 1, the electrical resistance and B-value at workingtemperature decrease in accordance with increase of a molar ratio ofLaCrO₃. It is an advantage of this invention that under practicalconditions of use, the resistance and B-value can be changed by changingthe molar ratio of spinel-type and perovskite-type components, asdescribed hereinbefore.

For the aging test under continuously applied d.c. voltage of 6V for1000 hours, it is desirable for practical use that the resistancedeviation be within 10 %. Such a small deviation is realized by thecompositions having more than 10 mole % of LaCrO₃. On the other hand,for the composition of 100 mole % of LaCrO₃, the B-value is too smallfor practical use. Therefore, the desirable composition of a hightemperature thermistor suitable for 400° to 600° C is limited to 10 to90 mole % of MgAl₂ O₄ and 10 to 90 mole % of LaCrO₃.

Furthermore, samples No. 14 to No. 20 in Table 1 show the effect of Srsubstitution for La ion in LaCrO₃ in a case of a principal compositionof 0.80 MgAl₂ O₄ +0.20 LaCrO₃. By this Sr substitution by amount from 5to 20 atom % for La ion, load aging characteristics are intensivelyimproved as shown in Table 1. The most effective results are provided atSr substitution of 16 atom %. For the compositions of the samples No. 2to No. 13 in Table 1, La ion is substituted by 16 atom % of Sr ion as insamples No. 22 to No. 33, and the same aging test was carried out. Theresults of the aging test are shown in Table 2. By these results of Srsubstitution for La ion, it is understood that the aging testcharacteristics of any principal compositions are intensively improved,although the other electrical characteristics are not so much improved.

For a thermistor composition of the invention for higher workingtemperature ranging from 600° to 1000° C, a ternary system compositionMgAl₂ O₄ --MgCr₂ O₄ --LaCrO₃ is available. Various compositions of thisternary system are shown in Table 3. It was found by X-ray analysis thatMgAl₂ O₄ and MgCr₂ O₄ actually formed a solid solution Mg(Al, Cr)₂ O₄ ina ceramic body. Therefore, this ternary system can be expressed by the(1-Z)Mg(Al₁ _(-x), Cr_(x))₂ O₄ --ZLaCrO₃ system. The manufacturingprocess for this thermistor is quite the same as that describedhereinbefore for the high temperature thermistor used at a workingtemperature range of 400° to 600° C. The electrical characteristics werealso measured in the same manner as that described hereinbefore. Butload aging test was carried out at 800° C under a continuously appliedd.c. voltage of 6V. The results of the measured electric characteristicsand aging test are summarized in Table 4.

From Table 3, it is understood that the resistance and B-value atworking temperature range can be widely changed according to atomicratio of Al to Cr in spinel-type Mg(Al, Cr)₂ O₄ and molar ratio ofspinel-type Mg(Al, Cr)₂ O₄ to perovskite-type LaCrO₃. This is anadvantage of this invention. As a rule, the resistance and B-valueincrease in accordance with increase of atomic ratio of Al to Cr inMg(Al, Cr)₂ O₄ and molar ratio of Mg(Al, Cr)₂ O₄ to LaCrO₃. Theresistance deviation at 800° C after 1000 hours under continuouslyapplied d.c. voltage by 6V should be within 10% for a practical use. Asshown in Table 4, the resistance deviation within 10% is satisfactorilyrealized by the composition of (1-Z)Mg(Al₁ _(-x) Cr_(x))₂ O₄ --ZLaCrO₃system; wherein

0.3≦x≦0.7 at Z=0.03

0.01≦x≦0.9 at Z=0.10

0.03≦x≦0.7 at Z=0.20

This composition area is expressed in FIG. 2 as the area enclosed by sixpoints a, b, c, d, e, and f in the ternary system A.MgAl₂ O₄ -- B.MgCr₂O₄ --C.LaCrO₃. In this case, the coordinates of these six points arecalculated as follows:

    ______________________________________                                        (a)       A=0.679      B=0.291      C=0.03                                    (b)       A=0.891      B=0.009      C=0.10                                    (c)       A=0.776      B=0.024      C=0.20                                    (d)       A=0.24       B=0.56       C=0.20                                    (e)       A=0.09       B=0.81       C=0.10                                    (f)       A=0.291      B=0.679      C=0.03.                                   ______________________________________                                    

Furthermore, Sr substitution for La ion in LaCrO₃ is also foundeffective to improve the load aging characteristics, similarly to theformer case. Table 5 shows the results of the measured electricalcharacteristics and aging test for Sr substitution for La ion incomposition of 0.99Mg (Al₀.7 Cr₀.3)₂ O₄ +0.01(La₁ _(-y) Sry)CrO₃, as anexample. As shown in Table 5, the results of the aging test areintensively improved, especially in case of Sr substituion by 5 to 20atom % for La ion in LaCrO₃. Table 4 also shows an effect of Srsubstitution for other compositions of different molar ratio of Mg(Al,Cr)₂ O₄ to LaCrO₃, as a case of Sr substitution by 16 atom. % for Laion. In Table 4, notations of A, B, C and D are used as terms ofappraisal for the load aging test characteristics as follows:

A: under 5 % resistance deviation after 1000 hours

B: under 10 % resistance deviation after 1000 hours

C: under 30 % resistance deviation after 1000 hours

D: above 30 % resistance deviation after 1000 hours.

It is found that even the compositions designated by C have smallresistance deviation after 1000 hours when the load aging test iscarried out at a rather lower temperature as from 400° to 600° C. Forinstances, the samples No. 43, 44, 51 and 92 showed a resistancedeviation within 10% when they were subjected to a temperature of 500° Cfor 1000 hours under continuously applied d.c. voltage of 6V. Thecompositions designated by A are preferred compositions for a hightemperature thermistor up to 1000° C, as they showed a resistancedeviation within 10 % after load aging test at 1000° C for 1000 hoursunder continuously applied d.c. voltage of 6V.

From these results, the thermistor compositions appropriate to a hightemperature thermistors to be used above 400° C are comprehensivelyobtained by a composition of the (1-Z)Mg(Al₁ _(-x) Cr_(x))₂ O₄ --Z(La₁_(-y) Sr_(y))CrO₃ system, wherein Z=0.45˜0.995, x=0.001˜0.95, andy=0.05˜0.20. This composition is expressed in FIG. 2 as the areaenclosed by a dotted line with four points a', b', c' and d' in ternarysystem MgAl₂ O₄ --MgCr₂ O₄ --LaCrO₃. The coordinates of these fourpoints are calculated as follows:

    ______________________________________                                        (a')     A=0.994     B=0.001     C=0.005                                      (b')     A=0.4495    B=0.0005    C=0.55                                       (c')     A=0.0225    B=0.4275    C=0.55                                       (d')     A=0.050     B=0.945     C=0.005.                                     ______________________________________                                    

                                      Table 1                                     __________________________________________________________________________                                               Resistance Deviation                                          Resistance                                                                                     ##STR1##                          Sample No.                                                                           Composition        at 400° C                                                                    at 800° C                                                                    B.value                                                                            after 1000 hours                   __________________________________________________________________________    1      MgAl.sub.2 O.sub.4 4.2×10.sup.12                                                                 1.0×10.sup.8                                                                  19200                                                                              50                                 2      0.98MgAl.sub.2 O.sub.4 +0.02LaCrO.sub.3                                                          3.4×10.sup.11                                                                 4.5×10.sup.7                                                                  16100                                                                              49                                 3      0.95MgAl.sub.2 O.sub.4 +0.05LaCrO.sub.3                                                          8.8×10.sup.10                                                                 1.25×10.sup.7                                                                 16000                                                                              41                                 4      0.90MgAl.sub.2 O.sub.4 +0.10LaCrO.sub.3                                                          1.2×10.sup.10                                                                 1.9×10.sup.6                                                                  15800                                                                              9.5                                5      0.80MgAl.sub.2 O.sub.4 +0.20LaCrO.sub.3                                                          5.6×10.sup.7                                                                  1.2×10.sup.4                                                                  12300                                                                              8.0                                6      0.70MgAl.sub.2 O.sub.4 +0.30LaCrO.sub.3                                                          7.3×10.sup.6                                                                  1.96×10.sup.4                                                                 10700                                                                              7.4                                7      0.60MgAl.sub.2 O.sub.4 +0.40LaCrO.sub.3                                                          9.3×10.sup.5                                                                  6.0×10.sup.3                                                                   9100                                                                              7.0                                8      0.50MgAl.sub.2 O.sub.4 +0.50LaCrO.sub.3                                                          2.9×10.sup.5                                                                  3.3×10.sup.3                                                                   8090                                                                              6.3                                9      0.40MgAl.sub.2 O.sub.4 +0.60LaCrO.sub.3                                                          6.5×10.sup.4                                                                  1.0×10.sup.3                                                                   7520                                                                              6.2                                10     0.30MgAl.sub.2 O.sub.4 +0.70LaCrO.sub.3                                                          6.4×10.sup.3                                                                  1.4×10.sup.2                                                                   4150                                                                              5.2                                11     0.20MgAl.sub.2 O.sub.4 +0.80LaCrO.sub.3                                                          5.7×10.sup.2                                                                  1.65×10.sup.2                                                                  2240                                                                              5.0                                12     0.10MgAl.sub.2 O.sub.4 +0.90LaCrO.sub.3                                                          3.1×10.sup.1                                                                  1.7×10.sup.1                                                                   1050                                                                              4.8                                13     LaCrO.sub.3        3.3×10.sup.0                                                                  2.47×10.sup.0                                                                  520 4.0                                14     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.99 Sr.sub.0.01)CrO.sub.3                                    5.5×10.sup.7                                                                  1.3×10.sup.4                                                                  12200                                                                              8.0                                15     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.98 Sr.sub.0.02)CrO.sub.3                                    5.3×10.sup.7                                                                  6.1×10.sup.4                                                                  12200                                                                              7.9                                16     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.95 Sr.sub.0.05)CrO.sub.3                                    5.3×10.sup.7                                                                  6.5×10.sup.4                                                                  12100                                                                              4.5                                17     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.90 Sr.sub.0.10)CrO.sub.3                                    5.1×10.sup.7                                                                  6.6×10.sup.4                                                                  12000                                                                              4.3                                18     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                    5.0×10.sup.7                                                                  7.2×10.sup.4                                                                  11800                                                                              2.8                                19     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.80 Sr.sub.0.20)CrO.sub.3                                    5.7×10.sup.7                                                                  6.2×10.sup.4                                                                  12300                                                                              3.6                                20     0.80MgAl.sub.2 O.sub.4 +0.2(La.sub.0.75 Sr.sub.0.25)CrO.sub.3                                    6.0×10.sup.7                                                                  5.9×10.sup.4                                                                  12500                                                                              15                                 __________________________________________________________________________

                                      Table 2                                     __________________________________________________________________________                              at 500° C                                    Sample No.                                                                           Composition        after 1000 hours, 16 atom % Sr.sub.Substitution     __________________________________________________________________________    22     0.98MgAl.sub.2 O.sub.4 +0.02(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            22                                         23     0.95MgAl.sub.2 O.sub.4 +0.05(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            15                                         24     0.90MgAl.sub.2 O.sub.4 +0.10(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            4.4                                        25     0.80MgAl.sub.2 O.sub.4 +0.20(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.8                                        26     0.70MgAl.sub.2 O.sub.4 +0.30(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.7                                        27     0.60MgAl.sub.2 O.sub.4 +0.40(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.7                                        28     0.50MgAl.sub.2 O.sub.4 +0.50(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.5                                        29     0.40MgAl.sub.2 O.sub.4 +0.60(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.4                                        30     0.30MgAl.sub.2 O.sub.4 +0.70(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.4                                        31     0.20MgAl.sub.2 O.sub.4 +0.80(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.2                                        32     0.10MgAl.sub.2 O.sub.4 +0.90(La.sub.0.84 Sr.sub.0.16)CrO.sub.3                                            2.1                                        33     (La.sub. 0.84 Sr.sub.0.16)CrO.sub.3                                                                       1.9                                        __________________________________________________________________________

                  Table 3                                                         ______________________________________                                        Sample No.  Composition                                                       ______________________________________                                        41          Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4                           42          Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4                           43          0.995Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.005LaCrO.sub.3                7                                                                 44          0.995Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.005LaCrO.sub.3                7                                                                 45          0.99Mg(Al.sub.0.999 Cr.sub.0.001).sub.2 O.sub.4 +0.01LaCrO.sub                .3                                                                46          0.99Mg(Al.sub.0.99 Cr.sub.0.01).sub.2 O.sub.4 +0.01LaCrO.sub.3    47          0.99Mg(Al.sub.0.9 Cr.sub.0.1).sub.2 O.sub.4 +0.01LaCrO.sub.3      48          0.99Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.01LaCrO.sub.3      49          0.99Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.01LaCrO.sub.3      50          0.99Mg(Al.sub.0.1 Cr.sub.0.9).sub.2 O.sub.4 0.01LaCrO.sub.3       51          0.99Mg(Al.sub.0.05 Cr.sub.0.95).sub.2 O.sub.4 +0.01LaCrO.sub.3    52          0.97Mg(Al.sub.0.999 Cr.sub.0.001).sub.2 O.sub.4 +0.03LaCrO.sub                .3                                                                53          0.97Mg(Al.sub.0.99 Cr.sub.0.01).sub.2 O.sub.4 +0.03LaCrO.sub.3    54          0.97Mg(Al.sub.0.97 Cr.sub.0.03).sub.2 O.sub.4 +0.03LaCrO.sub.3    55          0.97Mg(Al.sub.0.9 Cr.sub.0.1).sub.2 O.sub.4 +0.03LaCrO.sub.3      56          0.97Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.03LaCrO.sub.3      57          0.97Mg(Al.sub.0.3 Cr.sub. 0.7).sub.2 O.sub.4 +0.03LaCrO.sub.3     58          0.97Mg(Al.sub.0.1 Cr.sub.0.9).sub.2 O.sub.4 +0.03LaCrO.sub.3      59          0.97Mg(Al.sub.0.05 Cr.sub.0.95).sub.2 O.sub.4 +0.03LaCrO.sub.3    60          0.9MgAl.sub.2 O.sub.4 +0.1LaCrO.sub.3                             61          0.9Mg(Al.sub.0.999 Cr.sub.0.001).sub.2 O.sub.4 +0.1LaCrO.sub.3    62          0.9Mg(Al.sub.0.997 Cr.sub.0.003).sub.2 O.sub.4 +0.1LaCrO.sub.3    63          0.9Mg(Al.sub.0.99 Cr.sub.0.01).sub.2 O.sub.4 +0.1LaCrO.sub.3      64          0.9Mg(Al.sub.0.97 Cr.sub.0.03).sub.2 O.sub.4 +0.1LaCrO.sub.3      65          0.9Mg(Al.sub.0.9 Cr.sub.0.1).sub.2 O.sub.4 +0.1LaCrO.sub.3        66          0.9Mg(Al.sub.0.8 Cr.sub.0.2).sub.2 O.sub.4 +0.1LaCrO.sub.3        67          0.9Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.1LaCrO.sub.3        68          0.9Mg(Al.sub.0.6 Cr.sub.0.4).sub.2 O.sub.4 +0.1LaCrO.sub.3        69          0.9Mg(Al.sub.0.5 Cr.sub.0.5).sub.2 O.sub.4 +0.1LaCrO.sub.3        70          0.9Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.1LaCrO.sub.3        71          0.9Mg(Al.sub.0.1 Cr.sub.0.9).sub.2 O.sub.4 +0.1LaCrO.sub.3        72          0.9Mg(Al.sub.0.05 Cr.sub.0.95).sub.2 O.sub.4 +0.1LaCrO.sub.3      73          0.9Mg(Al.sub.0.01 Cr.sub.0.99).sub.2 O.sub.4 +0.1LaCrO.sub.3      74          0.8Mg(Al.sub.0.999 Cr.sub.0.001 ).sub.2 O.sub.4 +0.2LaCrO.sub.                3                                                                 75          0.8Mg(Al.sub.0.99 Cr.sub.0.01).sub.2 O.sub.4 +0.2LaCrO.sub.LaC                rO.sub.3                                                          76          0.8Mg(Al.sub.0.97 Cr.sub.0.03).sub.2 O.sub.4 +0.2LaCrO.sub.3      77          0.8Mg(Al.sub.0.9 Cr.sub.0.1).sub.2 O.sub.4 +0.2LaCrO.sub.3        78          0.8Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.2LaCrO.sub.3        79          0.8Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.2LaCrO.sub.3        80          0.8Mg(Al.sub.0.1 Cr.sub.0.9).sub.2 O.sub.4 +0.2LaCrO.sub.3        81          0.8Mg(Al.sub.0.05 Cr.sub.0.95).sub.2 O.sub.4 +0.2LaCrO.sub.3      82          0.7Mg(Al.sub.0.999 Cr.sub.0.001).sub.2 O.sub.4 +0.3LaCrO.sub.3    83          0.7Mg(Al.sub.0.99 Cr.sub.0.01).sub.2 O.sub.4 +0.3LaCrO.sub.3      84          0.7Mg(Al.sub.0.97 Cr.sub.0.03).sub.2 O.sub.4 +0.3LaCrO.sub.3      85          0.7Mg(Al.sub.0.9 Cr.sub.0.1).sub.2 O.sub.4 +0.3LaCrO.sub.3        86          0.7Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.3LaCrO.sub.3        87          0.7Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.3LaCrO.sub.3        88          0.7Mg(Al.sub.0.1 Cr.sub.0.9).sub.2 O.sub.4 +0.3LaCrO.sub.3        89          0.7Mg(Al.sub.0.05 Cr.sub.0.95).sub.2 O.sub.4 +0.3LaCrO.sub.3      90          0.6Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.4LaCrO.sub.3        91          0.6Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.4LaCrO.sub.3        92          0.45Mg(Al.sub.0.999 Cr.sub.0.001).sub.2 O.sub.4 +0.55LaCrO.sub                .3                                                                93          0.45Mg(Al.sub.0.99 Cr.sub.0.01).sub.2 O.sub.4 +0.55LaCrO.sub.3    94          0.45Mg(Al.sub.0.9 Cr.sub.0.1).sub.2 O.sub.4 +0.55LaCrO.sub.3      95          0.45Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.55LaCrO.sub.3      96          0.45Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.55LaCrO.sub.3      97          0.4 Mg(Al.sub.0.7 Cr.sub.0.3).sub.2 O.sub.4 +0.6LaCrO.sub.3       98          0.4Mg(Al.sub.0.3 Cr.sub.0.7).sub.2 O.sub.4 +0.6LaCrO.sub.3        ______________________________________                                    

                                      Table 4                                     __________________________________________________________________________                                Resistance                                                                              Resistance                                                          Deviation Deviation                                                           after 1000 hrs.                                                                         after 1000 hrs.                                Resistance (Ω) at 800° C (%)                                                                    at 800° C                         Sample No.                                                                           Temperature at 400° C                                                          Temperature at 800° C                                                          B-value                                                                            ##STR3##  16 atom. % Sr Substitution                                                              Appraisal                    __________________________________________________________________________    41     1.0×10.sup.7                                                                    3.4×10.sup.4                                                                    10300                                                                              510       150       D                             42     6.8×10.sup.3                                                                    2.2×10.sup.2                                                                     6190                                                                              870       330       D                             43     3.0×10.sup.6                                                                    1.3×10.sup.4                                                                     9820                                                                              98        30        C                             44     3.4×10.sup.3                                                                    1.2×10.sup.2                                                                     6040                                                                              105       29        C                             45     3.7×10.sup.10                                                                   4.6×10.sup.6                                                                    16200                                                                              100       21        C                             46     1.8×10.sup.9                                                                    1.2×10.sup.6                                                                    13200                                                                              95        19        C                             47     1.7×10.sup.8                                                                    3.4×10.sup.5                                                                    11200                                                                              71        17        C                             48     8.0×10.sup. 5                                                                   4.9×10.sup.3                                                                     9200                                                                              29        8.4       B                             49     2.3×10.sup.3                                                                    9.0×10                                                                           5850                                                                              44        9.0       B                             50     9.4×10                                                                          1.4×10                                                                           3440                                                                              78        16        C                             51     3.9×10                                                                          8.2      2820                                                                              105       28        C                             52     1.4×10.sup.10                                                                   2.4×10.sup.6                                                                    15700                                                                              24        7.1       B                             53     7.7×10.sup.8                                                                    6.4×10.sup.5                                                                    12800                                                                              13        4.5       A                             54     1.9×10.sup.8                                                                    3.3×10.sup.5                                                                    11500                                                                              4.6       2.0       A                             55     5.7×10.sup.7                                                                    1.6×10.sup.5                                                                    10600                                                                              4.1       2.0       A                             56     1.4×10.sup.5                                                                    1.9×10.sup.3                                                                     7760                                                                              3.8       1.8       A                             57     1.9×10.sup.3                                                                    7.2×10                                                                           5910                                                                              4.4       2.1       A                             58     6.4×10                                                                          1.0×10                                                                           3350                                                                              11        3.3       A                             59     3.2×10                                                                          7.6      2600                                                                              26        5.9       B                             60     1.2×10.sup.10                                                                   1.9×10.sup.6                                                                    15800                                                                              250       7.7       D                             61     4.5×10.sup.9                                                                    1.1×10.sup.6                                                                    15000                                                                              29        6.9       B                             62     1.4×10.sup.9                                                                    6.8×10.sup.5                                                                    13800                                                                              18        5.1       B                             63     3.7×10.sup.8                                                                    3.6×10.sup.5                                                                    12500                                                                              7         3.0       A                             64     1.5×10.sup.8                                                                    2.0×10.sup.5                                                                    12000                                                                              3.9       1.8       A                             65     5.1×10.sup.7                                                                    8.9×10.sup.4                                                                    11500                                                                              2.5       1.5       A                             66     1.9×10.sup.6                                                                    8.6×10.sup.3                                                                     9740                                                                              1.6       1.2       A                             67     2.7×10.sup.5                                                                    2.2×10.sup.3                                                                     8680                                                                              0.4       0.2       A                             68     5.4×10.sup.4                                                                    6.6×10.sup.2                                                                     7950                                                                              1.7       1.1       A                             69     1.4×10.sup.4                                                                    2.3×10.sup.2                                                                     7420                                                                              3.2       1.9       A                             70     1.8×10.sup.3                                                                    5.9×10                                                                           6170                                                                              4.3       2.5       A                             71     7.2×10                                                                          1.1×10                                                                           3390                                                                              8         3.6       A                             72     3.6×10                                                                          7.8      2760                                                                              19        4.1       A                             73     2.2×10                                                                          6.3      2260                                                                              190       43        D                             74     3.2×10.sup.7                                                                    7.1×10.sup.4                                                                    11000                                                                              30        10        B                             75     1.8×10.sup.7                                                                    5.9×10.sup.4                                                                    10300                                                                              14        5.1       B                             76     7.0×10.sup.6                                                                    3.7×10.sup.4                                                                     9460                                                                              4.6       2.0       A                             77     1.9×10.sup.6                                                                    1.8×10.sup.4                                                                     8410                                                                              3.3       1.8       A                             78     5.1×10.sup.4                                                                    9.4×10.sup.2                                                                     7210                                                                              2.5       1.8       A                             79     2.2×10.sup.2                                                                    2.7×10                                                                           3790                                                                              3.9       2.0       A                             80     4.7×10                                                                          8.0      3200                                                                              11        3.4       A                             81     2.6×10                                                                          6.2      2590                                                                              27        7.3       B                             82     1.3×10.sup.6                                                                    9.0×10.sup.3                                                                     8980                                                                              70        17        C                             83     1.7×10.sup.6                                                                    1.7×10.sup.4                                                                     8310                                                                              56        11        C                             84     7.8×10.sup.5                                                                    8.6×10.sup.3                                                                     8140                                                                              40        9.0       B                             85     2.1×10.sup.5                                                                    4.2×10.sup.3                                                                     7060                                                                              28        7.5       B                             86     1.2×10.sup.4                                                                    5.0×10.sup.2                                                                     5740                                                                              21        6.2       B                             87     6.0×10                                                                          1.1×10                                                                           3060                                                                              46        9.1       B                             88     3.2×10                                                                          6.0      3020                                                                              58        9.2       B                             89     2.1×10                                                                          5.6      2390                                                                              73        21        C                             90     3.1×10.sup.3                                                                    2.8×10.sup.2                                                                     4340                                                                              56        15        C                             91     2.5×10                                                                          5.4      2770                                                                              63        17        C                             92     3.4×10.sup.4                                                                    6.2×10.sup.2                                                                     7230                                                                              88        24        C                             93     6.0×10.sup.4                                                                    2.3×10.sup.3                                                                     5890                                                                              77        20        C                             94     1.2×10.sup.4                                                                    5.3×10.sup.2                                                                     5630                                                                              67        20        C                             95     1.6×10.sup.3                                                                    2.0×10.sup.2                                                                     3750                                                                              59        19        C                             96     1.7×10                                                                          4.3      2480                                                                              65        21        C                             97     7.8×10.sup.2                                                                    1.3×10.sup.2                                                                     3230                                                                              130       45        D                             98     1.0×10                                                                          3.7      1800                                                                              205       63        D                             __________________________________________________________________________

                                      Table 5                                     __________________________________________________________________________    Y        0    0.01 0.02 0.05 0.08 0.12 0.16 0.20 0.25 0.30                    __________________________________________________________________________    Characteristics                                                               Resistance                                                                             2.3×10.sup.3                                                                 2.3×10.sup.3                                                                 2.2×10.sup.3                                                                 2.1×10.sup.2                                                                 2.0×10.sup.3                                                                 1.8×10.sup.3                                                                 1.7×10.sup.3                                                                 2.1×10.sup.3                                                                 6.9×10.sup.3                                                                 4.0×10.sup.4      at 400° C                                                              Resistance                                                                             9.0×10                                                                       8.9×10                                                                       8.7×10                                                                       8.4×10                                                                       8.1×10                                                                       7.4×10                                                                       6.9×10                                                                       8.3×10                                                                       2.2×10.sup.2                                                                 6.2×10.sup.2      at 800° C                                                              B-value  5850 5840 5830 5800 5780 5760 5760 5810 6210 7500                    Resistance                                                                    Duration 44   44   39   22   14   12   9.0  15   79   185                     after                                                                         1000 hrs.                                                                     __________________________________________________________________________

What is claimed is:
 1. A high temperature thermistor compositioncomprising 10 to 90 mole % of spinel-type MgAl₂ O₄ and 10 to 90 mole %of perovskite-type LaCrO₃.
 2. A high temperature thermistor compositionaccording to claim 1, wherein La ion in said LaCrO₃ is substitutedpartially by 5 to 20 atom. % of Sr ion.
 3. A high temperature thermistorcomposition having the ternary system A.MgAl₂ O₄ --B.MgCr₂ O₄--C.LaCrO₃, wherein said composition is defined by the area of FIG. 2enclosed by following six composition points;

    ______________________________________                                        (a)       A=0.679      B=0.291      C=0.03                                    (b)       A=0.891      B=0.009      C=0.10                                    (c)       A=0.776      B=0.024      C=0.20                                    (d)       A=0.24       B=0.56       C=0.20                                    (e)       A=0.09       B=0.81       C=0.10                                    (f)       A=0.291      B=0.679      C=0.03.                                   ______________________________________                                    


4. A high temperature thermistor composition having the ternary systemA.MgAl₂ O₄ --B.MgCr₂ O₄ --C.(La₁ _(-y) Sr_(y))CrO₃, wherein y is definedas 0.05≦y≦0.2, wherein said composition is defined by the area of FIG. 2enclosed by following four composition points;

    ______________________________________                                        (a')     A=0.994     B=0.001     C=0.005                                      (b')     A=0.4495    B=0.0005    C=0.55                                       (c')     A=0.0225    B=0.4275    C=0.55                                       (d')     A=0.050     B=0.945     C=0.005.                                     ______________________________________                                    